Periscopes are used in a number of military applications to provide the occupants of a vehicle, such as a tank, with a view of the outside environment. In addition to the optical view from the periscope, such military vehicles are often also equipped with cameras that may provide digitally enhanced views of the outside environment, for example night vision or thermal imaging. It would be desirable to provide a periscope in which the user could switch between an external optical view and an electronically-generated view without changing the viewing position. Such a periscope is desirably relatively compact in order to allow retrofitting into existing vehicles without affecting the comfort or ease of exit of the user.